Intrinsic reinforcements for obtaining finer grains or higher hardness

additional reinforcements combined with the base alloy, the hardness value can be further improved and the grain size can also be refined.

different elements. After FSP, all thin foils are combined together and the materials in the stir zone are mixed well. The stir zone for the multi-element intermetallic specimen was typically about the size of the rotation pin, namely 6 mm in width and 5 mm in depth, which is slight smaller than the dynamically recrystallized zone seen in the AZ31 Mg alloys (~7 mm in width and 6 mm in depth). It is conceivable that the intermetallic specimens are much harder and become less deformable during FSP.

3.2.1.2 Microstructure

The alloy composites vary in a wide range within Mg37.5-80Al5-25Zn10-45. Figures 3-16 to 3-21 present some examples of the SEM micrographs using the backscattering electron image (BEI). For example, the microstructure of the Mg70Al5Zn25 system with air cooling after three passes of FSP, as shown in Fig. 3-16, contains a fine grained darker Mg (measuring 0.1-2 μm) surrounded by lighter Al and nearly white Zn. The low melting Zn (Tm~ 420^oC) appears to have fully melted during FSP and is squeezed into a thin layer coating the Mg grains. There are a few large particles with distinct contrast in Fig. 3-16, typically around 5 μm in size. The composition determined by SEM energy dispersive spectrometry (EDS) is close to Mg3Al2Zn3 (sometimes defined as the Mg32(Al,Zn)49) τ phase, with a cI162 space group and a cubic lattice constant of 1.42 Å when slowly cooled or with an icosahedral structure when rapidly quenched. For higher Al and Zn content, to compositions of Mg50Al5Zn45 or Mg37.5Al25Zn37.5, the τ particles occupy with higher volume fraction in the matrix, as shown in Figs. 3-17 and 3-18. With the cooling rate improved by copper clamping kits with a water cooling system, the microstructures also tend to be more homogeneous with smaller τ particles, as shown in Fig. 3-19. After ten FSP passes, the microstructure of Mg37.5Al25Zn37.5

becomes more and more homogenous with much smaller τ particles, as shown in Fig. 3-20.

Systematic SEM/EDS measurements for the white contrast phase have been conducted. It is

consistently found that the composition of this phase is close to Mg49.2±1.8Al17.2±1.5Zn33.6±1.8, not far from the Mg3Al2Zn3 τ phase, as determined by XRD and TEM diffractions. For comparison, the Mg70Al5Zn25 alloy system was also prepared by rapid quenching melt spinning with a cooling rate around 10⁵ K/s. The typical SEM/BEI micrograph is shown in Fig. 3-21. There is no apparent large intermetallic particle seen in the matrix. The whole specimen contains a homogenous and amorphous phase.

3.2.1.3 X-ray diffraction

The XRD patterns of the Mg70Al5Zn25 system fabricated by FSP and melt spinning are compared in Fig. 3-22. There is no detectable crystalline peak for the melt spun amorphous specimen, but many crystalline peaks for the FSP counterpart. The typical XRD patterns of the Mg70Al5Zn25, Mg50Al5Zn45 and Mg37.5Al25Zn37.5 alloys fabricated by FSP such are shown in Fig. 3-23. When the Al and Zn contents were low, the XRD patterns of the Mg80Al10Zn10

system resembled those of the FSP AZ31 alloys. With much more Al and Zn in Mg70Al5Zn25, Mg50Al5Zn45 and Mg37.5Al25Zn37.5, the XRD patterns in Fig. 3-23 are replaced by multiple phases, including the intermediate phases of Mg3Al2Zn3 (in the form of either cubic or icosahedral crystal structure), Mg2AlZn and Mg4AlZn11, and possibly some others. The diffraction peaks gradually declined and broadened with increasing FSP passes, suggesting the overall refinement of the microstructure. The peak broadening becomes more apparent in Mg50Al5Zn25 or Mg37.5Al25Zn37.5, as shown in Fig. 3-23. It appears that the increasing FSP passes and higher Al and Zn contents are promising means of refining grains to submicron- or nano-scales.

3.2.1.4 Hardness measurement

The typical hardness measurement results along the transverse cross-sectional plane are depicted in Fig. 3-24. The average Hv values in the various alloy systems and processes are also compared in Table 3-4. The hardness is seen to gradually increase with increasing Al and Zn contents (Fig. 3-24). The average hardness value approaches 350 Hv in the Mg50Al5Zn45

and Mg37.5Al25Zn37.5 systems after 3 passes without cooling system. However, the variation of hardness values with higher Al and Zn contents also slightly increases. Increasing the FSP passes would reduce the variation of Hv readings inside the nugget zone and the microstructure becomes more uniform. The hardness value of Mg37.5Al25Zn37.5 after 10 passes shows minimum variation. This is due to the highly uniform microstructure in this system which can be seen in the Fig. 3-20. The hardness value of Mg70Al5Zn25 fabricated by melt spinning could approach 264 Hv, but the hardness value of the same alloy system fabricated by FSP is only 164 Hv. The difference in hardness is originated from the different phase structures. It is obvious that a higher cooling rate is still necessary to generate fully or partially amorphous phase in the Mg-Al-Zn system.

3.2.1.5 TEM examination

TEM characterization on the Mg37.5Al25Zn37.5 alloys, based on both diffraction patterns and TEM/EDS measurements, has revealed the abundant Mg3Al2Zn3 phase. In addition to the large particles (~3-10 μm) seen from both the SEM and TEM micrographs, there are many submicron- or nano-sized intermetallic phases, as shown in Fig. 3-25. The diffraction and EDS analyses suggest that these are mostly the Mg3Al2Zn3, Mg7Zn3, Mg2AlZn phases. The intermediate Mg3Al2Zn3 particles, defined as the τ phase, have been seen to possess either the cubic or icosahedral five-fold symmetry point group, the latter is shown in Fig. 3-25(c). Both phases have the composition close to Mg3Al2Zn3, but would be formed upon slow or rapid cooling. During FSP, both phases have been induced, suggesting the cooling rate after the pin

stirring is within the overlapping region for these two phases. The grain sizes of the elemental Mg as seen in TEM micrographs are typically in the range of 0.05-1 μm, which are also smaller than those seen in SEM. It means that the Mg phase observed in the SEM/BEI micrographs is actually composed of several submicron or nano-sized grains. This result signified the good grain refinement efficiency of FSP. Meanwhile, there are also some nano-scaled intermetallic phases with sizes near ~10-100 nm dispersed inside the Mg grains.

3.2.1.6 Brief conclusions of in-situ formed intermetallic compounds reinforced Mg-Al-Zn alloys made by FSP

Bulk intermetallic compounds reinforced Mg-Al-Zn alloys with different fractions of AZ31 sheets, Al and Zn foils were successfully fabricated by friction stir processing.

Multi-passes and high fractions of Al and Zn elements results in apparent grain refinement, proved by the broadening of diffraction peaks and from SEM results. Some intermetallic compound phases were generated after multi-passes FSP and some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the multi-element Mg base alloy made by FSP reached nearly 350 in Hv scale, especially in the Mg50Al5Zn45 or Mg37.5Al25Zn37.5 system, due to the generation of intermetallic compounds and grain refinement. The microstructure and hardness in the stirred zone become much more refined and uniform due to increasing FSP passes to ten.

Although the in-situ formed imtermetallic compound reinforced Mg-Al-Zn alloys can possess extremely high hardness and some nano-grains or particles, the original compositions of pure AZ31 alloy are shifted a lot. The applicable properties for in-situ formed imtermetallic compound reinforced Mg-Al-Zn alloys are also different with the AZ31 alloys.

Therefore, using the extrinsic reinforcements for obtaining finer grains or higher hardness is

taken into consideration. The composition of particle reinforced Mg based metal matrix composites can remain closer to the original base alloy and not be shifted much. The applicable properties of this kind of composite are more comparable with the original base alloy.